Laser printers are designed to produce an image on paper or other print material by placing overlapping dots at virtual positions defined by a digital raster. These virtual positions in the raster are known as “pixels”, short for “picture elements”. In laser printers, the image is typically first produced on a charged photoconductive material by scanning the photoconductor with a laser beam in a sequence of scan lines. As a line is scanned, or written, the laser beam is energized during the extent of each pixel that corresponds to a dot in the image. When the laser beam is energized, it discharges the portion of the photoconductor upon which the laser energy impinges. The sequential scanning by the laser produces a charge-pattern copy of the desired image on the photoconductor. The printed output is obtained by developing the charge pattern by attracting toner to it and transferring the developed image to paper or other print material.
Color laser printers use four laser-photoconductor developer systems to simultaneously expose, develop, and print four different color planes (e.g., cyan, yellow, magenta, and black, or “CYMK”) to produce a colored print. These types of color printers are known as “in-line” color printers. For an in-line printer to produce a high quality image, proper alignment must be maintained between the scan lines of each of the four color planes. Improperly aligned color planes degrade print quality by producing visual artifacts such as fringing of colors and poorly defined edges, similar to a poorly adjusted color TV or a badly printed copy of color newspaper comics. However, due to the cumulative affects of varying mechanical tolerances between the four separate laser/photoconductor units, each of the four color planes almost inevitably varies slightly from the other three. A slight divergence between the four color planes will almost certainly reduce print quality by producing a color “halo” somewhere in the print image.
Generally, imaging hardware on a laser scanning device provides a pixel data stream at a laser writing frequency to a laser driver that converts the pixel data stream to a laser control signal which, in-turn, is converted by a laser unit into a laser beam that is scanned along a scan line. Attempts to improve the alignment between the four color planes can potentially cause instability in pulse width modulator (PWM) type laser drivers. This instability can last for several cycles while the PWM recovers and returns to stable operation. Consequently, during a recovery period a PWM may provide a distorted output signal which can result in a corresponding distortion in an image produced by a laser unit on a print material.